About 450 runners are expected to be at the starting line when the gun sounds at 11 a.m. today to begin the 16th annual Washington's Birthday Marathon at the U.S. Department of Agriculture Research Center in Beltsville.

The 26-mile 385-yard race will start at the Log Lodge Cafeteria, about two miles east of Rte. 1 on Powder Mill Road. The contestants will travel three times around a loop of approximately nine miles before the race ends back at the cafeteria.

Bob Rothenberg, who will coordinate the event for the Road Runners' Club of America, said yesterday that Max White of Alexandria and Carl (Ed) Hereford of Williamstown, N.C., have the best shot at the course record of 2 hours 24 minutes 17 seconds set in 1973.

Bill O'Brien of Buffalo, N.Y., who captured the race last year in 2:26:40, is not expected to return. But Carolyn Hahn of Washington, whose time of 3:25:40 was the fastest among the women last year, has filed an entry.

Rothenberg said that runners may register by paying the $3.50 entry fee at the cafeteria during the hour before the start of the race.

All entrants will be served a free meal after the race and all finishers will be awarded a race tee-shirt. About 60 trophies and merchandise prizes will be presented to the top finishers.

A three-mile, run-for-fun race will begin at the cafeteria at 10 a.m.